Volume limiter circuit



March 27 1945 P. G. wuLFsBERG VOLUME LIMITEE CIRCUIT 4 sheetsnsheef. 2

Filed Sept.- 4, 1942 B (55o v) c) 9 w" V120m/T012. BY4

ATTOR/VFY March 27,1945. P. G. WULFSBER'G 2,372,243Y

VOLUME LIMITEE CIRCUIT Filed Sept. 4, 1942 4 Sheets-$116013 l N l? 5% w I'OOO March'27, 1945. 'j P. G. WULFSBERG 2,372,243

VOLUME LIMITER CIRCUIT Filed Sept. 4, 1942 4 Sheets-Sheet 4 5 K55 LIMITERM! Patented Mar. 27, 1.945 I VOLUME LIMITEE CIRCUIT Paul Gerhard Wulfsberg, Cedar Rapids, Iowa, as-

signor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application September 4, 1942, Serial No. 457,370

9 Claims. (Cl. 178-4-1) My invention relates broadly to volume limiting electron tube amplifier circuits and more particularly to an improved limiting amplifier circuit arrangement which is adaptable to resistance coupled amplifiers.

One of the objects of my invention is to provide a balanced bridge type of volume limiting amplifier circuit that is adaptable to a. resistance coupled amplifier with the elimination of audio frequency transformers.

Another object of my invention is` to provide an improved arrangement for a volume limiting circuit in which transient currents, incident to a change in attenuation in the circuit, are balanced out by the establishment of counter E. M. F.s which do not aiiect the passage of the desired current.

Still another object of my invention is to provide a current limiting amplifier including a tube constituted by double .triodes each having independent control grids arranged in separate circuits with a fixed bias applied to the control grid `of one trode and a variable bias applied to the control grid of the associated triode and with the'coacting anodes connected in balanced rela- 'tion for delivering to an output system controlled current.

A further object of my invention is to -provide `a volumevlimiting amplifier circuit arrangement in which a pah of electron .tube discharge systems are arranged to form the arms of a Wheatstone bridge circuit in association with balanc- 'ing resistors constituting the complementary arms of the bridge circuit with an input circuit connected across one portion of the Wheatstone bridge circuit and an output circuit connected across another portion of the Wheatstone bridge circuit and with the tubes in the arms thereof operating to balance the Wheatstone bridge circuit for effectively controlling the attenuation of energy impressed Aupon the input circuit and :aenvered to the output circuit while substannauy transient currents in the amplifica- .:tionfisystem which are incident to a sudden .change in the plate current (and therefore the plate resistance) of the tube.

' Other and further objects of my invention reside in the balanced Wheatstone bridge circuit arrangement as set forth more fully in the speciiication hereinafter following by reference to the panying drawings, in which:

I Figure 1 is a theoretical diagram showing the 4i'i'indamental circuit of the volume limiting cir- I cuit-,of 'my invention;

Fig. 2 is a diagrammatic "vlewfishowing one form of electron tube circuit 'amusement the principles of my 1.11-

vention: Fig. 3 is a chart showing the characteristics of the particular tube which has been used in the volume limiting circuit of my invention; Fig. 4 is a diagrammatic circuit arrangement of one of the forms of the volume limiting circuit of my invention; Fig. 5 shows a double Wheatstone bridge circuit and balancing amplifier system constructed in accordance with my invention and showing the means employed for balthe volume limiting circuit of Fig. 4; Fig. 9 is al chart showing the frequency response characteristics ofthe volume limiting circuit of my invention; and Fig. 10 shows a modified circuit arrangement embodying the principles of my invention in a push-pull or double-bridge volume limiting circuit.

Figure 1 shows the fundamental Wheatstone circuit employed in the system of vmy invention in which the four resistances R1, Rz, Ra and R4 comprise the four arms of the bridge. Assuming that the audio frequency transformer 2 is without transmission loss, the power loss from the input `1 circuit 3'to the output circuit 4 is a function of the ohmic values of the resistances R1, R2, Ra and R4 in the bridge. If R1 R4=R2 Ra, the loss in the circuit-is infinite, whereas if one of the resistance values in the bridge is very large compared to the other three, the loss in the circuit may be small.

In the circuit illustrated in Fig. 2, the resistance arms R3 and R4 have been supplanted by electron tubes V1 and V2 whose plate resistances serve as the third and fourth arms of the bridge. The input terminals are represented at 3 connected-to the bridge circuit through condenser 6 whlethe output terminals are shown at 4 connected through audio frequency transformer 2 in the output circuits of tubes V1 and V2. In order to provide the tubes V1 and V2 with plate voltage, modifications of the circuit in Fig. 1 are necessary. The tubes V1 and V2 in Fig- 2 are shunt fed from a source of plate potential through resistance Rs. Resistance R5, if made sufiiciently large, will not causemuch additional loss in the source of xed bias. The term no compression refers to the point a't which the circuit ls intended to operate normally. At this point the rectified voltage from the control tube would have no eiect on the plate current of the tube Vs.

When there is no compression in the circuit. the

grid Vig of tube Vn is at a potential more negative than the grid V1: of tube Vi and the plate current of tube V: is therefore very small since the voltage across bias resistor Re also biases the tube V2. Bias resistor Re is shunted by condenser 1. The value of resistor Re is such that the bias on the grid V1. of tube Vi is suillcient to limit the plate current to a normal value. From Fig.` 2 it may be seen that changes in grid or plate voltages in one tube will anect theother because of the common cathode circuit l. Compression in the circuit begins to take place as the grid of Vn; of tube V: becomes less negative in potential. This increases the plate current in tube V2, causing its plate resistance to drop. An increase in the plate current of tube V: causes the bias on the cathode Vic oi tube Vi to increase, thereby decreasing the plate current of tube V1, but increasing its plate resistance. As the plate resistance of tube Va decreases toward the value of the plate resistance o! tube V1, or the plate resistance of tube V1 increases toward that of A tube Va, the bridge will approach a balanced condition, assuming that R1=Rz, and the power loss through the circuit will increase rapidly.

Fig. 3 shows the plate resistance characteristic of the type 6C8G tube I have used in this circuit. The 6C8G tube consists of two low mu triodes in one glass envelope. A represents the normal operation point of triode Va and "B represents the normal operating point of triode V1. As the` compression in the circuit is increased, the plate resistance of each of the triodes moves toward the point indicated by "C. At this point the plate resistances of tubes V1 and V2 are equal and, assuming R1=Rz, the circuit has an iniinite loss in the ideal case, but practically this 5 value is about 45 to 50 db. with the particular circuit constants which have been employed.

Fig. 4 shows the circuit of my invention for which the data set forth herein is given. The electron tube constituting'the equivalent oi tubes V1 and V2 is indicated at 8 containing within the same envelope the cathodes Baand 8a', the coacting control grids 8b and 8b' and the coacting anodes 8c and 8c. The cathodes 8a and la' are connected together and to the biasing resistor 2 shunted by condenserl and connected to ground at I0. The input terminals 3 connect through the network Il comprising condensers i2, Il and I4 and resistors I5 and I8 connected as shown. The network is connected to the arms of the bridge through resistors l1 and IB (R1 and R2) .terminating at anodes 8c and llc'. The circuit to the cathodes from the network Il is completed through resistors I9 and 20 connected in series with intermediate tap 2| extending to control grid 8b for establishing the fixed bias. The control grid 8b is shown connected to tap 22 in the resistance circuit containing' resistors 23 and 24, the resistor 24 being variable so that the gain may be varied manually in place of a rectier circuit from the audio output circuit as would normally be. employed. The variable resistor 24 is connected to ground at 25. The output circuit of the Wheatstone bridge which is thus established extends from anodes 8c and 8c' to the transform-,

- 9,872,248 4grid encarece v1. of tube v1, is connected to a er 2 .having its primary winding 23 connected through condenser 21 with anodes 8c and Bc' and its secondary winding 28 connected to output terminals 4 with regulating resistance 29 connect- 5 ed in shunt therewith.

It will be observed that the xed bias is applied to control grid lb While variable bias is applied to control grid 8b'. The graph for the response data obtained from this circuit is shown l in F18. 9. In this circuit it was found that sudden changes in compression caused slight low frequency transients in the output circuit due to the change in D. C. potential between the anodes Ic and lc of tubes Vi and V2.

Fig. shows a double bridge circuit for a balanced input which hasrthe same characteristics as theA circuitf in Fig. 4 except that no transients were observed in this circuit. In this circuit the D.l C. pulses are balanced out since theyv set up counter E. M. F.s `in the two primary windings I0 and Il of the output transformer 32. The arrangement of tubes is similar to the arrangement set forth in Fig. 4 except that two such double triode tubes are provided. I have shown these tubes at 33 and 34, each including cathodes 33a and Ila'; control grids 33h and 33b'; coacting anodes 33e and 32e in tube 33;.and in tube 34 cathodes 34a and 34a; control grids 34b and 24b'; and anodes 34e and 34e. Bias voltage is 80 provided for the grid electrodes by the cathode current through resistor 9 shunted by condenser l and connected to ground I0 in a manner similar to a biasing arrangement in the cir- .cuit of Figs. 2 and 4. The input circuit termi- 35 nals 3 are connected through coupling condensers 35 and BB-to the resistance network including resistors 31 and 38 connected through tap 39 to condenser which is connected to ground at si. A connection is completed from one end of the network through resistors 42 and 43 to anodes #3c land 33c' respectively. A connection is completedfrom the other end of the network through resistors 44 and 45 with anodes 34e and 34e as shown. The input circuits are completed through resistors I9 and 20 connected to ground lll in a manner similar to the arrangement illustrated in Fig. 4 and in a similar way`a lead from tap 2| is extended to control grids 33h and 34h as shown. The circuit from control grids 33h' and 34h' is completed in a manner similar to that illustrated in Fig. 4. Resistor 23 and variable resistor 24 are provided connected tothe positive side of the source oi' B potential and including the .iiitgi'mediate tap 22 and the connection to ground The circuit includes as heretofore pointed out, two separate output systems including a path from anode 33c through primary winding 30 and m condenser 4S, returning to anode 33e'. The complementary primary winding 3| is included in a circuit connected at one end to anode 34e and extending through condenser 41 returning to anode 24e'. 'I'he counter E. M. F.s in the prig5 mary windings 30 and '3| balance outI transient currents transferring only the currents impressed at 2 and desired at the terminals 4 connected to the secondary winding 28 ot transformer 3'2 to 1- which the two primary windings 20 and 3l are coupled.

` Fig. 6 shows the volume limiting circuit in which the input and output connections are reversed from those shown in Figs. 4 and 5. This circuit requires a balanced input voltage but re- "u quires' no output transformer.

It has charac.

- AE represents the s sponse curves for teristics very similar to those of the circuit shown in Fig. 4. A tube similar to the tube represented in Fig. 4 is employed containing cathodes 8a and 8a', control grids 8b and 8b' and coacting anodes 8c and 8c. The cathodes 8a and 8a are connected through biasing circuit including resistor 9 and shunt condenser 1 to ground IIJ and to the potentiometer circuit including resistor I9, 28, variable resistor 48 -and resistor 49 connected to the B supply circuit between terminal 50 and ground I as shown. The anodes 8c and 8c are connected through resistors I1 and I8 to the positive potential supply circuit through regulating resistance 5| as shown. The input circuit at 3 is balanced and is connected through condensers 52 and 53 with anodes 8c and 8c respectively. The output circuit requires no output transformer but is connected to output terminals 4, one terminal of which is connected to ground as shown at l0.

Fig. 7 is a theoretical diagram showing the equivalent A. C. circuit for the circuit in Fig. 4.

the primary winding 28 of transformer 2.

Fig. 8 is a vectordiagram showing the relation of the currents in the circuit of Fig. 4 where Eo designates the input voltage; Ei the voltage across arm Ri (11); E2 the voltage across arm R2 (18); E3 the voltage across arm Ra (V1) and E4 the voltage across arm R4 (V2) AE represents the voltage across the primary winding 28 of the transformer 2.

Measured values of the voltages of the circuit in Fig. 4 (see Figs. '1 and 8) are as follows for several operating conditions. The equivalent plate resistance for each tube is also given:

Amount 0f C0111- Eo E1=EUE3 Eg=EurE4 V E3 E4 AE pression Volts Volta Volts Volta Volts Volts 0 db 2. 0 0. 86 0. 17 1. 14 1. B3 0. 67 db 2.0 0.88 0.50 .1.12 1.50 0.37 io cbm.- 2.o 0.89 0.61 1.11 1.33 0.22

Calculated Calculated Amount of compression pltcgeifst' plggecegfst' tube Vi tube V:

Ohms Ohms Fig. 9 shows the characteristic frequency rethe volume limiting circuit of my invention in which curve 55 shows the frequency response of the volume limiting. circuit for a condition of no compression; curve 58 shows the characteristic of the frequency response in the volume limiting circuit for 5 db. compressionyand curve 51 shows the frequency response characteristicsfor the volume limiting circuit of 10 db. compression. It will be seen that the balanced circuit arrangement operates to give substantially iiat frequency response over a relatively broad range of frequencies.

In Fig. 10 I have shown a modied form of my invention in which a push-pull or double-bridge volume limiting circuit is provided. A single duotriode 58 is provided including cathode 58a, control grid 58h and anode 58e and also including cathode 58a', control grid 58h and anode 58o. The cathode bias illustrated in Fig. 5 is replaced by a iixed grid bias so that there is no longer a through condenser 54 voltage which appears across audio frequency transmission systems.

needl'or tubes in both arms of the bridge circuit as previously required. The control rectifier as illustrated at 59 includes the cathodes 59a and 59a' and associated anodes 59h and 59o having input transformer 80 with primary winding 6| and magnetically coupled secondary windings 62 and 63 connected to the anodes 59h and 59h respectively and connected to the source of negative bias voltage through lead 84. The windings 62 and 83 are mutually connected vat 65 and connect through resistor 88 with the cathodes 59a and 59a which connect to ground through condenser 81. The primary winding 6I connects in the output circuit of the audio frequency amplifier system 88, the volume 0f whose output is to be liml ited. A connection is made to one side of the primary winding 8| to the sourceof B potential indicated at 89 through plate supply resistor 10. A connection is made to ground around plate supply resistor 10 through condenser 1|. The amplifier system 68 is shown leading to the succeeding stage through coupling condenser 12. Normal grid bias is provided through cathode resistor 13 shunted by condenser 14. The input circuit to the limiter tube 58 is similar to that described in connection with Fig. 5, that is, the input terminals 3 connect through coupling condensers 35 and 35 to the resistance network including resistors 31 and 38 and the associated resistors 15, 18, 11 and 18. The anodes 58c`and 58e' are connected with the network through resistors 19 and 80. The cathodes 58a and 58a.' connect to ground at 8| and to tap intermediate resistors 16 and 11 at 82 through condenser 83 to tap 84 intermediate resistors 31 and 38 to lead f 85, resistor 88, to the positive side of the B potential source indicated at 89. The primary winding 30 of transformer 32 is connected at one end with anode 58o and at the opposite terminal with tap 81 intermediate resistors 15 and 16. The primary winding 3l of transformer 32 connects at one end to anode tap 88 intermediate resistors 11 and 18.

The circuit including the control rectifier has been found to be entirely satisfactory and eicient in operation. The several forms of the volume limiting circuit of my invention have been successfully used in the control of current in While I have described my invention in certain preferred embodiments, I realize that changes may be made by those skilled in the art in the details of the circuit arrangement and it is not my intention that my invention be limited to the precise various modifications of the circuits illustrated and various frequencies including radio frequencies within the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows: 1. A volume limiting circuit for operation at audio frequencies comprising a pair of electron discharge devices, each of said devices including a pair of triodes. each comprising a cathode, a control grid -and ,a coacting anode and forming the arms of a pair of Wheatstone bridge circuits, balancing resistors connected with said anodes and constituting the complementary arms of each of the Wheatstone bridge circuits, an input circuit common to said pair of Wheatstone bridge circuits, separate output circuits connected with the anodes of each of said devices, and an output circuit connected in common to said separate output circuits.

58o' and at the other end to 'odes of each of said Wheatstone .means for impressing a variable stone bridge circuits v said triodes .and-the 2. .A volume limiting circuit for operation at audio frequencies comprising a pair of electron discharge devices, each of said devices including a pair of triodes, each comprising a cathode, a control grid and a coacting anode and forming the varms of a pair of Wheatstone bridge circuits.. balancing resistors constituting the complementaryA arms of each of the Wheatstone bridge circuits and electrically connected with said anodes, an input circuit common to said pair of Wheatstone bridge circuits, anl output circuit including inductive windings connected in shunt with the anodes of the triodes in the respective Wheatstone bridge circuits, said inductive windings being coupled in opposed relation for cancelling transient currents in said circuits, and an output winding coupled with each of said inductive windings.

3. A volume limiting circuit foroperation at audio frequencies comprising a pair of electron discharge devices, each of said devices including a pair of triodes, each comprising a cathode, a control grid and a coacting anode and forming the arms of a pair of Wheatstone bridge circuits,

balancing resistors constituting the complementary arms of each of the Wheatstone bridge circuits and electrically connected with said anodes, an input circuit common to said pair of Wheatstone bridge circuits, a circuit including inductive windings and condensers disposed in series and terminating at the anodes of therespective triodes in said Wheatstone bridge circuits, said inductive windings being coupled in opposed relation for cancelling transient currents incident upon said Wheatstone bridge circuits, and an output winding coupled with each of said inductive.

windings for receiving attenuated current substantially free from transients.

4. A volume limiting circuit for operation at audio frequencies comprising a pair-or electron discharge devices, each of said devices including a pair of triodes, each comprising a cathode, a control grid and a coacting anodeand forming the arms of a pair of Wheatstone bridge circuits,

balancing resistors constituting the complementary arms of each of the Wheatstone bridge circuits and electrically connected with said anodes, an input circuit .common-to said pair of Wheatstone bridge circuits, means iorA impressing a xed bias on the lcontroigrid of bridge circuits, bias upon the control grids of the other triodes of each of said Wheatstone bridge circuits, an-output circuit each includingan inductance and condenser disposed in series and connected with each of said Wheatstone bridge circuits, and means coupling said output circuits for cancelling transient currents therein while transferring the current impressed upon the input circuit.y

.5.'A volume circuit for operation at audio frequencies comprising an electron discharge system constitutedby a pair oi' triodes. each of said triodes including a cathode, a control grid and a coacting anode, a pair of Wheatstone bridge circuits, one arm of each of said Wheatbeing constituted b'y one of other armsoi said Wheatstone-bridge circuits being constituted by balancinc resistors Vcomie'cted with the anodes of said triodes, an input circuit common to each of said Wheatstone bridge circuits, an output circuit connected with the anode of the triode of one Wheatstone bridge circuit and with the balancing. resistors of that circuit, a separate output circuit one of the triconnected with the anode of the `other of said triodes in the other of said Wheatstone bridge circuits and with the balancing resistors in said last mentioned Wheatstone bridge circuit, and a common output circuit coupled with each of said separate output circuits.

6. A volume limiting circuit for operation at audio frequencies comprising an electron discharge system constituted by a pair of triodes, each of said triodes including a cathode, a control grid and a coacting anode, a pair of Wheatstone bridge circuits, one arm of cach of said Wheatstone bridge circuits being constituted by one of said triodes and the other arms of said Wheatstone bridge circuits balancing resistors, an input circuit common to each of said Wheatstone bridge circuits, an output circuit connected with the anode of the triode of one Wheatstone bridge circuit and with the balancing resistors of that circuit, a separate output circuit connected with the anode of the other of said triodes in the other of said Wheatstone bridge circuits and with the balancing resistors in said last mentioned Wheatstone bridge circuit, and means in said output circuits for substantially cancellingY transient currents in said lWheatstone bridge circuits while transferring the current impressed upon the input circuit and desired at the output circuit.

7. A volume limiting circuit for operation at audio frequencies comprising an electron discharge system constituted by a pair of triodes, each of said triodes including a cathode, a control grid and a coacting anode, a pair of Wheatstone bridge circuits, one arm of each of said Wheatstonebridge circuits being constituted by the electron discharge path through one of said triodes and the other arms of said Wheatstone bridge circuits being constituted by balancing resistors, an input circuit common to each of said Wheatstone bridge circuits, an output circuit connected with the anode of the triode of one Wheatstone bridge circuit and with the balancing resistors of that circuit, 'a separate output circuit connected with the anode of the other of said triodes in the other of said Wheatstone bridge circuits and with the balancing resistors in said last mentioned Wheatstone bridge circuit, means `for impressing a. predetermined bias potential upon each of said control grid electrodes, and means coupled with both of said output circuits forv transferring current from said output charge system constituted the electron ldischarge path connected with the anode of triodes in the other orsaid Wheatstone bridge circuits substantially free of 8. A volume hunting circuit for operation at audio frequencies comprising an electron disby a pair ofA triodes. each of said triodes including a cathode, a cony trol grid and a coacting anode, a pair of Wheatstone bridge circuits, one

arm of each of said being constituted by through one of said triodes and the other arms of said Wheatstone bridge circuits being constituted by balancing resistors, an input circuit common to each of said Wheatstone bridge circuits, an output circuit connected with the anode of the triode of one Wheatstone bridge circuit and with the balancing resistors of that circuit, a separate output circuit the other of said Wheatstone bridge circuits circuits and with the balancing resistors in said last mentioned Wheatstone bridge circuit, a common source of bias potential, means for impressing said bias potential on each of said control grid electrodes, and means for inductively -anbeing constituted by transient currents.

nuling the eiect of transient currents generated in said Wheatstone bridge circuits incident to varying-the attenuation of the current impressed upon the input to the circuits.

9. A volume limiter circuit for operation at audio frequencies comprising a duo-triode tube including two sets of cathode control grid and anode electrodes, an input circuit connected 1n symmetrical arrangement with said anodes and cathodes, a resistance network connected in shunt with the cathode and anode of each of the sets of electrodes, a control rectifier disposed in circuit with said control grid electrodes for iin-l pressing bias potential thereon, a pair of output tap in said resistance network for each ot said 'rectifier for controlling sets o! electrodes, a primary winding in each of said output circuits, a. secondary winding coupled to both of said primary windings, an audio frequency ampliiier coupled with said secondary winding and a circuit extending from the output of said audio frequency amplifier to said control the operation of said control rectiiier and governing the bias potential impressed upon the grid electrodes of said duotriode tube.

PAUL GERHARD' WULFSBERG. 

